1. The Field of the Invention
The present invention relates to a circuit for monitoring voltages of batteries in a telephone terminal facility, and more particularly to a monitoring circuit for detecting voltages of the batteries in a telephone terminal facility only when a handset of a telephone is picked up or a ringing signal is received, in which both consumed power and voltage variations are low.
2. Description of the Related Art
A modern smart telephone set or a telephone or an answering machine with a function of distinguishing a caller's identification code (a caller's ID), etc. has batteries provided therein to supply the electrical power required for preventing the stored data being lost due to a power failure. Furthermore, such telephone equipment has a power detecting circuit installed therein for detecting voltages of the batteries to avoid an abnormal operation caused by insufficient battery power. When the batteries need to be changed, a user is informed by a display or an indicator lamp which shows the lack of electrical power when the battery voltage is not enough. Presently, however, in the power detecting circuit used for detecting the voltages of the batteries in a telephone terminal facility, there exist problems of an excessive consumption of the electrical power or an incorrect operation which are described as follows.
A conventional power detecting circuit used in a telephone terminal facility is a circuit which operates all the time. Not only does this power detecting circuit waste electrical power due to its continuously consuming the electrical power, but also the other circuits in the telephone terminal facility are also operating when the telephone terminal facility is in a normal operating state and this easily causes a noise signal to be generated in the electrical power due to variation of electrical loads. These will be liable to result in a misdetermination by the power detecting circuit. To focus on this problem, although the battery voltages are repeatedly detected many times during a certain interval for some products to avoid the misdetermination described above, a battery voltage-detecting operation performed in an intermittent way, however, requires a counter or timer and thus increases both the consumption of the electrical power and the noise signal generated in the electrical power during a detecting period instead. Therefore, apparently, the design of the conventional power detecting circuit can not meet the requirements of saving the electrical power and accurately detecting the voltages of the batteries and thus needs to be improved.